A generic LDR would work.
Yes the rectifier is there to supply DC from the AC line.
And yes you could replace the lamp with a LED, although it may not work quite as well. just change R24 to around 600ohms.
You could probably replace the 2n2712's with any high gain NPN, may I suggest 2n2222, 2n3904 or 2n4401 if you can't find 2n2712.